Add basic ADC support

Planning on abstracting this behind hardware setup APIs, but this works
for a simple demo until then

routines/photosensor.c

@@ -0,0 +1,27 @@
+#include <zephyr/kernel.h>
+
+#include "includes/ble.h"
+#include "includes/routine.h"
+#include "includes/hardware/adc.h"
+
+routine_return_e
+photosensor()
+{
+    while (routine_status() == ROUTINE_STATUS_RUNNING) {
+        int32_t adc_mv = adc_get_mv(0);
+        char buf[16];
+        size_t print_len;
+
+        if (adc_mv == INT32_MIN) {
+            ble_set_telemetry_data("Reading failed", 14);
+        } else {
+            print_len = snprintk(buf, sizeof(buf), "%dmV", adc_mv);
+            ble_set_telemetry_data(buf, print_len);
+        }
+
+        routine_delay(500);
+        routine_yield();
+    }
+
+    return ROUTINE_RETURN_SUCCESS;
+}